[6.16.z] Add donwload cuncurrency for repository

[Satellite-QE]

Cherrypick of PR: #1272

Description of changes

Add donwload cuncurrency for repository

Upstream API documentation, plugin, or feature links

https://apidocs.theforeman.org/katello/latest/apidoc/v2/repositories/create.html

Functional demonstration

log output:

2025-02-11 09:34:21 - nailgun.client - DEBUG - Making HTTP POST request to https://...katello/api/v2/repositories with options {'auth': None, 'verify': None, 'headers': {'content-type': 'application/json'}}, no params and data {"name": "test-uln", "content_type": "yum", "download_policy": "on_demand", "url": "uln://ovm2_2.1.1_i386_patch", "upstream_username": "***", "upstream_password": "***", "download_concurrency": 1, "organization_id": 8, "product_id": 304}